There has recently been a great deal of interest in developing better and more efficient materials for storing energy for applications such as radio communication, satellites, portable computers, and electric vehicles, to name but a few. Accordingly, there have been recent concerted efforts to develop high energy, cost effective battery cells having improved performance characteristics. Electrochemical battery cells are preferred and hence are widely used in these applications since the chemical reactions which take place in the cells can be converted into useful electrochemical energy.
An electrochemical battery cell uses its reactive components, namely the anode and cathode, to generate an electric current. The electrodes are separated from one another by an electrolyte which maintains a simultaneous flow of ionic conduction between the two electrodes. Electrons flow from one electrode through an external circuit to the other electrode completing the circuit. Rechargeable, or secondary, cells are more desirable than primary (non-rechargeable) cells since the associated chemical reactions are reversible. Accordingly, electrodes for secondary cells must be capable of being regenerated (i.e., recharged) many times. The development of advanced rechargeable cells depends on the design and selection of appropriate materials for the electrodes and the electrolyte.
Currently, materials selected for use as the anode in rechargeable electrochemical cells are fairly limited. Typically, most rechargeable electrochemical cells use either cadmium or a combination of three or more materials in a so-called metal hydride system, as the anode. However, both of these commonly used systems have shortcomings. For example, cadmium while widely accepted in the marketplace, has certain environmental issues associated with it. Specifically, cadmium is a highly toxic material and is difficult to dispose of. In fact, several countries have recently adopted legislation aimed at eliminating the use of cadmium in rechargeable cells.
Metal hydrides are environmentally benign, but have certain technological problems relating to performance. For example, cycle life (i.e. the number of times the cell can be discharged and charged) in the best metal hydride systems is typically less than 500 cycles, and normally approximately 300 cycles. In addition to relatively short cycle life, other problems such as short shelf life, hydrogen outgassing, and high internal pressures are inherent in the system and are difficult to overcome.
Accordingly, there exists a need to develop a new electrode material for rechargeable electrochemical systems, which is environmentally friendly, yet overcomes the limitation of prior art systems.